A touch sensor, which is a device that detects a position pointed at using a finger or pen, etc., or whether a pointing operation has been performed, is usually used in combination with a flat-panel display device such as an liquid crystal display (LCD) or plasma display (PDP). The output of the touch sensor is input to a computer to control an apparatus by the computer or control what is displayed on a flat-panel display device, whereby an easy-to-use man-machine interface is realized. Touch panels find practical use in game machines, PDAs, ticket machines, automatic teller machines (ATMs) and car navigation.
Analog capacitive-coupled systems, resistive-film systems, infrared systems, ultrasonic systems and electromagnetic induction systems are known as touch-sensor systems. Among these, the analog capacitive-coupled systems are classified further into those of projected capacitive and surface capacitive types. A surface-capacitive touch sensor comprises a transparent substrate, a uniform transparent conductive film formed on the substrate and a thin insulating film formed on the conductive film. When the touch sensor is driven, AC voltage is applied from the four corners of the transparent conductive film. If the touch sensor is touched by a finger, very small currents flow into the finger owing to a capacitance formed by the surface of the touch panel and the finger. These currents flow from the respective corners to the point touched. A controller determines the ratios of these currents and computes the coordinates of the position touched. With regard to techniques relating to surface-capacitive touch sensors, Patent Document 1 discloses a basic apparatus, and a well-known example relating to this is disclosed in Patent Document 2. The latest technical trends in analog capacitive-coupled systems are disclosed in Non-Patent Document 1.
In analog capacitive-coupled touch sensors of the past, a surface-capacitive touch sensor formed on a transparent substrate and a flat-panel display device are used in superimposed form. With such an arrangement, however, the fact that the touch sensor exists on the display surface results in certain problems, namely an increase in the thickness of the device itself, a rise in cost and a decline in display quality. Techniques for solving these problems are disclosed in Patent Documents 3 and 4.
Patent Document 3 discloses a device in which current detectors are attached to the four corners of a common electrode that impresses voltage upon liquid crystal and the positional coordinates of a touched portion are computed based upon currents that flow into the four corners. Patent Document 4 discloses a device comprising a liquid crystal display circuit for supplying a transparent opposing electrode with a display voltage or current; a position detecting circuit for detecting currents that flow from a plurality of locations of the transparent opposing electrode; and a switching circuit for forcing either one of these circuits into electrical conduction with the transparent common electrode. In accordance with both of these documents, the common electrode or the transparent opposing electrode performs the role of a surface-capacitive transparent conductive film and the display device need not be provided separately with a surface-capacitive touch sensor. As a result, it is maintained that the problems of an increase in the thickness of the device itself, a rise in cost and a decline in display quality are solved.
[Patent Document 1]
U.S. Pat. No. 4,293,734
[Patent Document 2]
Japanese Patent Kokai Publication No. JP-A-56-500230
[Patent Document 3]
Japanese Patent Kokai Publication No. JP-P2003-99192A
[Patent Document 4]
Japanese Patent Kokai Publication No. JP-P2003-66417A
[Patent Document 5]
Japanese Patent No. 3121592
[Non-Patent Document 1]
Yuji Mitani, Supervisor, “Technologies and Developments of Touchpanels”, CMC Publishing, Dec. 27, 2004